The present invention relates to a lens element for progressive spectacles, having at least one near viewing zone and a distance viewing zone which have different focusing powers, the lens element having a predetermined prismatic power.
Furthermore, the present invention relates to a method for producing a lens element having at least one near viewing zone and a distance viewing zone which have different focusing powers, and a predetermined prismatic power.
Spectacle lenses for correcting visual deficiencies are generally known. In particular, it is also known to produce lens elements for spectacles which have a multifocal optical power. These serve the purpose, in particular, of correcting the age-related visual deficiency, that is to say a reduction, caused by age-related phenomena, in the accommodative power of the eye.
In ophthalmic fabrication, a spectacle lens which attains prescribed dioptric powers at beam paths running through prescribed measuring and reference points at said points is produced starting from a dioptric prescription which is determined, for example, by an ophthalmologist. The position of the measuring and reference points depends on the type of lens, for example whether a single-vision or multifocal lens is involved, and is determined by the manufacturer.
With reference to the terminology used below and the definition of its respective meaning, reference is made to Standard DIN EN ISO 13 666, in which the vocabulary of ophthalmology is standardized. Here, DIN stands for “Deutsche Institut für Normung e.V.” [German Standards Institute], EN for “Europäische Norm” [European Standard] and ISO for “Internationale Organisation für Normung” [International Standards Organization]. Where they are not explicitly otherwise defined below, the meanings defined in the standard hold. In particular, the definitions of the following terms are found there: near viewing and distance viewing design point as well as near viewing and distance viewing reference point or near design reference point and distance design reference point (Chapters 5.13 and 5.14), progressive surface (7.7), progressive (spectacle) lens or progressive-power lens, progressive-addition lens (8.3.5), distance portion and distance viewing zone (14.1.1), near portion and near viewing zone (14.1.3), progression channel or intermediate corridor (14.2.11), addition (14.2.1), thickness reduction prism or prism thinning (14.2.11) and prism reference point (14.2.12).
Progressive spectacles are distinguished, in particular, in that each lens element has at least two zones with different focusing actions, which are designed for viewing at different object distances. By contrast with conventional multifocal lens elements, in which an abrupt transition occurs between the different zones, progressive lens elements are distinguished by the fact that the focusing action changes continuously between the zones. Progressive spectacles are therefore “smooth”, that is to say the functions of the surfaces of the front and back sides of the lens elements are twice continuously differentiable. In general, a lens element for progressive spectacles includes a zone situated in an upper part of the lens for viewing at relatively large object distances, a so-called “distance viewing zone”, and a zone therebeneath for seeing at relatively small object distances, a so-called “near viewing zone”. The terms “above” and “below” relate in this case to the orientation of the lens element when it is worn by a user. Between the near viewing zone and the distance viewing zone lies a transition zone of continuous change in power, the so-called “progression zone”.
Here, the manufacturer determines respectively both for the distance and for the near viewing zone a measuring point at which the dioptric power of the respective zone can be controlled, the so-called “distance design point” and “near design point”, respectively. He also determines yet a further point at which the prismatic power of the lens element is to be realized and can be controlled, the so-called “prism reference point”.
The prismatic power at the prism reference point is determined from the prescribed prismatic power and a thickness reduction prism. The thickness reduction prism is a prismatic power with a vertical base direction, that is to say the thickness reduction prism leads only to a beam deflection in a vertical direction without a horizontal deflection, in order to reduce the thickness of the lens element of the progressive spectacles, and not to change the horizontal component of the prismatic power. The thickness reduction prism must be the same in both lens elements of a pair of spectacles. The total prismatic power resulting from the prescribed prismatic power and the thickness reduction prism can then be designed either for a beam path used by an actual spectacle wearer, or for a beam path in a measuring instrument.
The dioptric power at a point on the spectacle lens is composed of the focusing action and the prismatic power. The focusing action consists of the spherical (sph) and the astigmatic powers, the astigmatic power including the cylinder strength (or the astigmatic difference cyl) and the axis direction (A). The prismatic power consists of the prismatic deviation (Pr) and the base setting (B). Consideration of the focusing and prismatic powers over the points of an entire zone of the lens element indicates a close relationship between the two powers. This relationship is described in simplified form by the Prentice formula, for example. Thus, a focusing action, constant in a relatively large zone, of a lens element of a spectacle lens leads to a specific continuous change in the prismatic power in this zone. Consequently, in the case of standard lens elements for progressive spectacles which have a spherotoroidal prescription surface and a progressive surface which remains unchanged for a specific range of prescribed powers, the prismatic power is permanently prescribed only at one point of the lens element, since (given a prescribed progressive surface) the required spherotoroidal prescription surface can in general not be used to attain prismatic powers prescribed at more than one point in the lens element. However, even with individually optimized lens elements a permanent prismatic prescription at more than one point generally leads to large additional spherical or astigmatic aberrations.
The prism reference point, that is to say the point at which the prismatic power is permanently prescribed, is preferably situated at a location which is used chiefly for seeing. Thus, if one's look is removed from this prism reference point, the difference between the actual prismatic power and the prismatic power at the prism reference point is enlarged with increasing distance. The reason for this is the focusing action of the lens element of the spectacle lens.
Consequently, in the case of anisometropia, that is to say given different dioptric prescriptions for the two eyes of a spectacle wearer, the prismatic powers at mutually corresponding points of the left and right lens elements of a pair of spectacles are generally not the same. Thus, if the prism reference point is in the distance viewing zone, given anisometropia similar prismatic powers for corresponding points in the distance viewing zone certainly result in the immediate surroundings of the prism reference point but, in return, larger prismatic differences result in the near viewing zone. These prismatic differences in the near viewing zone are then enlarged in a fashion approximately proportional to the spherical difference in power between the prescriptions for the right and left eyes. The situation is similar for a position of the prism reference point in the near viewing zone.
It is known that the prismatic power can be split in vector terms into a horizontal component and a vertical component. This is known, for example, from the specialist article entitled “Astigmatische and prismatische Einstärken-Brillengläser” [“Astigmatic and prismatic single-vision spectacle lenses”], Alfred Schikorra, reprint from the special periodical “Der Augenoptiker”, Willy Schrickel publishing house, Leinfelden, 1987. It is therefore known that given a position of the prism reference point in the progression zone, or else at the transition from the distance viewing zone to the progression zone, the horizontal component of the prismatic power in the distance viewing zone does not generally correspond to the horizontal component of the prescribed prismatic power. This difference results chiefly from the tilted position of use of the lens element in front of the eye of a spectacle wearer, or from an astigmatic prescription. It is particularly pronounced for lenses having an astigmatic prescription and an oblique axis position. However, this then forces the spectacle wearer to make an unnatural movement of convergence or divergence of his pair of eyes when looking through the distance viewing zone, and this not only causes an unpleasant feeling during wearing, but can also possibly cause consequential damage over a lengthy period.
For progressive lenses with an astigmatic prescription of ≦1 dioptres, the document U.S. Pat. No. 7,216,977 B2 proposes a continuous increase in the horizontal prismatic power along a main line of sight such that the difference between the horizontal prismatic power at the distance and near reference points is at least 2 dioptres. However, this additional requirement placed on the progressive lens results in large additional spherical or astigmatic errors for the spectacle wearer, since this mode of procedure presupposes many additional horizontal prismatic requirements along the main line of sight. Moreover, the document referred to does not give any details as to how the vertical component of the prismatic power is to be realized.
Document FR 2 814 819 A1 proposes a progressive lens having a horizontal prism which supports a convergence of the eyes and is present either only in the near viewing zone, over the entire progressive lens, or in a fashion rising from the distance viewing zone to the near viewing zone along the main line of sight. There is no examination of the effects of this horizontal prismatic requirement on the other aberrations, on the prismatic powers for the spectacle wearer in the distance viewing zone of the progressive lens, nor on the realization of a possibly prescribed prism.
Document EP 1 412 806 B1 proposes to add a vertical prism to a lens in order to improve the image quality of the lens. However, there is no examination of changes in the horizontal component of a prismatic power.
Document EP 1 107 849 B1 proposes introducing an additional prism in a near viewing zone in order to guide the optical center as close as possible to the center of the near viewing zone, that is to say the object in this document is to improve the near viewing zone by means of a vertical prism. However, there is no examination of changes in the horizontal component of a prismatic power.
Document DE 698 13 803 T2 describes the use of a horizontal prism to correct a peripheral zone of a lens element with a high degree of bending. This can also include a surface correction in the viewing zone in order to minimize optical errors. However, in this case there is no examination of the relationship between a plurality of prismatic requirements placed on an individual lens element and the losses, typically resulting therefrom, in the imaging quality. Nor is there an examination of the specific requirements for progressive lenses.
It is therefore an object of the present invention to provide an improved lens element for progressive spectacles.